Hi In my system I use sql server 2005 enterprise edition. My db is about 100 gb. I analyzed with DTA and created indexes that is needed but performance didn't improve. when I look execution plan I see a lot of table spools, costant scan, nested loop. what are these and how can I get rid of them? besides I think I have also a problem with recompilation. could you help me about how to prevent recompition?
Welcome to the forum! You're asking a very broad question without providing much information about your actual problems. The execution plan operators you've mentioned are explained here on the site in multiple articles. Just look around and read. You'll find that the only operator to maybe worry about is a table spool and even that is not necessarily always true. Also, check out these MS whitepapers: Troubleshooting Performance Problems in SQL Server 2005 Batch Compilation, Recompilation, and Plan Caching Issues in SQL Server 2005
Addition to what Frank mentioned, a great e-book from Red-gates on Execution plans (by Grant Fritchey) is also recommended. Try this: http://www.red-gate.com/about/book_store/sql_server_execution_plans.htm